This invention is concerned with techniques for determining the difference between two optical images.
The inherent parallelism of optics (a beam of light can carry different information on various portions of the light beam without interference) and the wide bandwidth which an optical system offers for communication make these systems ideal for applications such as real-time image processing, optical interconnection schemes, and associative processing. As a result, optics is emerging as an area of increasing importance in high-speed information processing. Real-time image processing is of particular interest in fields, such as robotics, which require the recognition and tracking of objects. A clear advantage to the optical approach for these applications is the capability of parallel processing, with its concomitant increase in processing speed over digital computing techniques. Additional applications for optical image processing include industrial quality assurance, optical logic gates, and the detection of motion in a scene.
The subtraction of two images was first proposed by Gabor, et al. (Physics Letters, Volume 18, Page 116 (1965)) by using a successive recording of the two images on a holographic plate with a 180 degree phase shift between the two and a subsequent readout of the composite hologram. This method requires the addition of a 180 degree phase shift, which must remain uniform throughout the two dimensional region within which the images overlap during the second recording. Image subtraction can also be achieved by other techniques, most of which require that a phase shift of 180 degrees be introduced between the two images. Recently, time reversal in phase conjugation was used to provide such a phase shift and the subtraction of two intensity patterns using a phase-conjugate Michelson interferometer was demonstrated (Chiou and Yeh, Phase-Conjugate Interferometric Coherent Image Subtraction, U.S. Pat. No. 4,718,749). These methods in the prior art, however, are limited in their applicability to cascaded operations and reflective objects because of their requirement for round-trip propagation.